Video: Bulk Density: Definition & Calculation

In this lesson, we'll discuss bulk density, a property of soils. You'll learn the definition of bulk density as well as how to calculate it. We'll also explore the differences between bulk density and other types of density.

Earned my B.S. in Civil Engineering back in 2011. Have two years of experience in oil and gas fields and two year as a graduate research assistant. Earned my Master degree in Engineering from Tennessee State University in 2016.

In this lesson, we'll discuss bulk density, a property of soils. You'll learn the definition of bulk density as well as how to calculate it. We'll also explore the differences between bulk density and other types of density.

Bulk Density

Bulk density, or dry bulk density, is a property of soils and other masses of particulate material. It's the weight of the particles of the soil divided by the total volume. Thus, it should be noted that the unit of bulk density is the unit of weight over the unit of volume, for example kg/m3 for the metric system and lb/ft3 for the English system.

Calculating Bulk Density

Bulk density is given by the following equation:

Let's look at an example:

A dry sample of soil has a volume of 0.5 m3 and weighs 800 kg. What's the bulk density of the sample?

Remember that:

Bulk density = mass of soil / total volume

Let's plug in our figures:

800 kg / 0.5 m3 = 1600 kg/m3

So, the bulk density of our soil sample is 1,600 kg/m3.

Here's another example:

A soil sample has a mass of solids equal to 400 lbs. and a water content of 20%. What is the bulk density of the soil given that the volume of the sample is 5.0 ft3?

To obtain the mass of water of the soil sample, we should multiply the water content and the solid mass:

Mw = 20% * 400 lb = 80 lb

Therefore, the total mass is:

400 lb + 80 lb = 480 lb

We can obtain the bulk density of the sample of the soil with our formula:

480 lb / 5.0 ft3 = 96 lb/ft3

Compaction

Have you ever emptied a container of spices to mix them in a bigger bowl, only to find that you couldn't place the spices back in their original container? The reason for that is that the spices were compacted by the production factory. When you emptied the container, the spices maintained their weight but increased in total volume because the voids' volume increased.

To calculate the total volume of soil, we add the volume of solids plus the volume of water plus the volume of air. The volume of voids is found by adding the volume of water and the volume of air. When a sample of soil is compacted, its volume of voids decreases which decreases the total volume of the soil sample. This increases the bulk density of the soil. So as depth increases, bulk density increases.

When there is moisture content in the soil, the wet bulk density is obtained by dividing the mass of solids plus the mass of water by the total volume.

Bulk Density vs Particle Density

As we learned earlier, the bulk density of a soil sample is the mass of the soil divided by the total volume of the soil. However, there's another property of the soil known as particle density. Particle density is the mass of solids in the soil divided by the volume of solids in that soil. That being said, we can see that the particle density does not increase or decrease when soil is compacted: it stays the same because compaction of a soil sample decreases the volume of voids but not the volume of solids.

Lesson Summary

Bulk density, or dry bulk density, is a property of soil obtained by dividing the mass of solids in the soil by the total volume. Wet bulk density, on the other hand, is obtained by dividing the mass of water plus the mass of solids by the total volume. This property can be increased by decreasing the total volume, and this can be done by decreasing the volume of voids by compaction.

Bulk density is not to be confused with particle density, which is obtained by dividing the mass of the solids in the soil by the volume of the solids in the soil. This means that the particle density cannot be increased by compaction since compaction doesn't change the volume of the solids.

Summary:

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